Effect of temperature variation on shift and broadening of the exciton band in Cs3Bi2I9 layered crystals

Abstract

Articles you may be interested inCommunication: Strong excitonic and vibronic effects determine the optical properties of Li2O2High direct energy band gaps determination in In x Al 1−x As coherently grown on InPThe exciton reflection spectra of Cs 3 Bi 2 I 9 layered crystals are investigated in the temperature region 4.2-300 K with light polarization EЌc. It is estimated that the energy gap E g equals 2.857 eV (Tϭ4.2 K) and the exciton binding energy Ry is 279 meV. A nontraditional temperature shift of E g (T) for layered substances is found for the first time. It is learned that this shift is described very well by the Varshni formula. A transition region in the temperature broadening of the half-width H(T) of the exciton band with increase of temperature is registered in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. A surge of H(T) at the point of the ferroelastic phase transition (T c ϭ220 K) is also observed.

Exciton-phonon interactions in the Cs3Bi2I9crystal structure revealed by Raman spectroscopic studies

“…Moreover, in accordance with Ref. , the peak at ∼2.6 eV was observed in the imaginary part of the complex dielectric function spectrum, directly linked to the resonant optical excitation of Cs 3 Bi 2 I 9 obtained at 4.2 K. This result is in good agreement with the maximum value of the ratio ( I 88 K / I 300 K ) 146 cm −1 at 2.6 eV, which overlaps with great accuracy the imaginary part of the dielectric function situated in the spectral range of 2.4–2.7 eV. The Raman spectrum at RT under an excitation light of 561 nm displays three main lines: (i) in the range of the low‐frequencies, a line situated at ∼62 cm −1 corresponds to the elementary layer packing of the [Bi 2 I 9 ] 3− vibration mode, which is associated with A 1 + symmetry; (ii) a line at ∼104 cm −1 , attributed to the [BiI 6 ] 3− vibration mode inside of the double octahedron package of [Bi 2 I 9 ] 3− ; and (iii) a line at ∼146 cm −1 , attributed to the vibration modes of the Bi–I bonds inside the [BiI 6 ] 3− octahedron . The vibration of the Cs + ions that link to the molecular anionic package has no contribution to the Raman spectrum, as the unit cell of Cs 3 Bi 2 I 9 contains isolated molecular packages of Bi 2 I 9 3− anion that seem to be very stable .…”

“…The main optical properties of Cs 3 Bi 2 I 9 are shown in Fig. and are as follows: The diffuse reflection spectrum at RT of the Cs 3 Bi 2 I 9 micrometric powder illustrated by the Kubelka–Munk function F ( R ) = (1– R 2 )/2 R with the reflectance R , reveals an inflection point at 1.87 eV, which indicates the limit of the bandgap energy and shifts towards higher energy at low temperatures . A higher value of the bandgap of Cs 3 Bi 2 I 9 has also been observed recently in a thin film obtained by using the spin‐coating method, and in a powder sample synthesized by solvothermal reaction, when bandgap values of 2.2 and 1.9 eV , respectively, have been reported.…”

From Lead Halide Perovskites to Lead‐Free Metal Halide Perovskites and Perovskite Derivatives

“…[120] The VBM consists mainly of I 5p orbitals slightly hybridized with Bi 6s orbitals, while the CBM is derived from the antibonding states of Bi 6p and I 5p orbitals. [118,[123][124][125][126] Figure 11c shows the Tauc plot of Cs 3 Bi 2 I 9 crystals at 10 K. Notably, the sharp excitonic peak at 2.56 eV is well separated from the electronic bandgap at 2.86 eV, corresponding to an excitonic binding energy, E b X = 300 meV. [118,121,122] The 0D structure also causes large exciton binding energies.…”

“…[118,121,122] The 0D structure also causes large exciton binding energies. [118,[123][124][125][126] Figure 11c shows the Tauc plot of Cs 3 Bi 2 I 9 crystals at 10 K. Notably, the sharp excitonic peak at 2.56 eV is well separated from the electronic bandgap at 2.86 eV, corresponding to an excitonic binding energy, E b X = 300 meV. [125] Both the E g and E b X in Cs 3 Bi 2 I 9 are higher than that in simple 2D layered BiI 3 (E g = 2.0 eV and E b X = 180 meV, respectively).…”

Spectroscopic studies of 2H‐PbI2(Mn) layered crystals

“…It increases at T < 40 K and decreases at T > 40 K, with approximately the same coefficient dE ex /dT (given for the above temperature regions in Table 1). This temperature behaviour of E ex (T) can be explained on the basis of essential influence of both anharmonic vibrations of the layered lattice (starting at low temperature) and low-frequency optical phonons (starting at higher temperatures) [6].…”